Laser printers have become increasingly popular with consumers due to their superior performance and rapidly declining price tags. These devices typically employ light beam sources such as laser diodes in order to generate highly focused beams of light that are used in order to print images onto a print medium such as paper.
The basic principles of monochromatic laser printers and color laser printers are well known and are similar. For example, single beam monochromatic laser printers will typically employ, in addition to a light beam source, various optical components and at least one transfer medium where an electrostatic latent image of the desired image to be printed is typically formed. In such devices, the light beam source (e.g., laser diode) is used to initially “draw” an electrostatic latent image (herein “latent image”) onto the transfer medium. More particularly, the highly focused light beam generated by the light beam source may be directed to the surface of the transfer medium via the printer's optical system (which typically includes the light beam source, one or more lens, and one or more mirrors), in order to create the many scan lines that make up an electrostatic latent image.
The transfer medium is usually a photosensitive drum or a photosensitive belt that comprises a photosensitive or photoconductive material that will discharge negative electrical charges in the form of static electricity when exposed to photons. By using a highly focused light beam generated by a light beam source, a latent image may be formed on the transfer medium. In general, the transfer medium is usually shorter than the actual image, so after toner is attracted to it, the photosensitive material needs to move the toner onto a transfer belt that is as long as the longest image or to the media itself. If the transfer is to an intermediate belt, all four colors of toner may be transferred to the belt prior to transferring to the media (usually paper). There is also the possibility of transferring the toner directly from the photosensitive material to the media and thus, no intermediate transfer belt would be needed. Such an embodiment will move the paper past the photosensitive material 4 times. The intermediate transfer belt and the media usually do not rely on being photosensitive to transfer the toner that is imaged on to the photosensitive material.
Generally, a ‘Top of Page’ generated from a sensor will usually be some location on the intermediate transfer belt or the edge of media itself detected along the paper path (the path the media travels throughout the printing process). It is usually not on the transfer medium since it is shorter (or the drum has a smaller circumference) than the media so a full image cannot reside on the transfer medium and will need to be transferred to something else. As used herein, transfer medium may include the transfer medium itself and an intermediate belt and/or the media itself.
In order to form an individual scan line of a latent image on the transfer medium, the optical system will initially direct the light beam to a location on the surface of the transfer medium, then move the light beam along, for example, a straight line on the surface of the transfer medium until the beam reaches an end point of the straight line. As the beam is moving along the straight line, certain characteristics of the beam may be selectively controlled and adjusted, such as light intensity, in order to create the individual pixels that make up the scan line. Once the light beam reaches the end of the straight line, a scan line is formed and the light beam source may then be turned off or at least powered down at least momentarily.
The transfer medium may then be incrementally moved and repositioned relative to the optical system, and more particularly, moved relative to the field of view of the optical system such that the optically system, which is generally stationary with respect to the transfer medium, may begin forming a new scan line that will typically be formed adjacent to the previously formed scan line. For example, if the transfer medium is a photosensitive drum, the drum may be incrementally rotated each time the optical system finishes forming a scan line. While the drum is being incrementally rotated, the optically system may be reconfigured, for example, by reconfiguring its mirror, which may be a rotating polygon scanning mirror, in order to begin forming a new scan line.
After all of the scan lines of the latent image have been formed, the latent image on the transfer medium may then be directly or indirectly used in order to print the desired image onto the print medium. That is, in some systems, the electrostatic latent image formed on the transfer medium may be used to directly print the desired corresponding image onto the print medium. Alternatively, the electrostatic latent image may be transferred onto a second, and even a third, transfer medium that may ultimately be used in order to print the desired image onto the print medium. If the transfer medium is used to directly print the corresponding image onto a print medium, then a toner in the form of a positively charged black ink power may be applied to the transfer medium, the toner only clinging to the negatively charged latent image. The powdered toner clinging to the latent image may then be rolled onto the print medium, which is then heated to fuse the toner onto the print medium.
Although the above technique for creating a monochromatic image uses a single light beam source, multi-beam systems are also known in the art in which multiple light beam sources (e.g., multiple laser diodes) are used in order to concurrently form adjacent scan lines of the latent image. Such systems are sometimes referred to as “multi-beam” systems and are typically used, for example, to print monochromatic images.
As for color laser printers, the process for printing color images in such devices is slightly more complicated than the process described above for monochromatic laser printers. In some implementations, color laser printers will have components similar to those of monochromatic laser printers including a light beam source, optical components, at least one transfer medium, and black ink toner. However, in addition to having the black ink toner, such devices will typically include other color toners, which are, but not always, cyan, magenta, and yellow. In order to print a color image, the transfer medium may pass through the printer's optical system (or more particularly, pass through the field of view of the optical system) multiple times in order to create the different color planes that will together be used in order to generate the desired color image. Such a process is sometimes referred to as “multi-pass.”
To further illustrate the multi-pass concept, during a first pass of the transfer medium through the optical system, a first color plane (e.g., black) of the latent image may be formed onto the transfer medium using the light beam source. During a second pass of the transfer medium, a second color plane (e.g., cyan) may be formed onto the transfer medium. During subsequent passes of the transfer medium, other color planes (.e.g., magenta and yellow) may also be formed onto the transfer medium. These color planes are typically formed one on top of the other to create the color latent image.
Due to certain economic and performance needs, multi-beam/multi-pass color laser printer systems have recently been proposed. However, alignment issues may arise when multi-beam techniques are used together with multi-pass techniques that are typically not encountered in multi-pass systems that employ a single beam. That is, when misalignment of the different color planes of an image occurs, which is generally referred to as registration errors, it can cause fringing and blurring of the resulting color image. Unfortunately, the use of multiple beams in multi-pass systems adds significant complexity and increases the likelihood as well as the scope of registration errors.